This invention relates generally to a technique for converting round logs into lumber products by cutting the logs into pieces which are then joined together, and more particularly to a technique in which the logs are cut into feedstock pieces having an isosceles trapezoidal cross-section, the pieces being fitted together in a complementary manner to create uniform layers which are superposed to form a stack defining a block assembly, the pieces in the block assembly being interlaminated to form an integrated stock block that is dividable into usable panels.
A technique in accordance with the invention, though applicable to various species of wood, is of particular value in connection with balsa wood derived from a tropical American tree (Ochroma pyramidale). Balsa wood has outstanding properties unique in the lumber field; for on the average, it weighs less than 9 pounds per cubic foot, this being 40% less than the lightest North American species. Its cell structure affords a combination of high rigidity and compressive and tensile strength superior to any composite or synthetic material of equal or higher density. While a technique in accordance with the invention will be described herein only in regard to balsa wood, it is to be understood that it is also applicable to many other wood species.
The market potential for balsa wood board is considerable; for structural sandwich laminates can be created by bonding thin facings or skins to balsa wood panels which function as a core. Thus the Kohn et al. U.S. Pat. Nos. 3,325,037 and the Lippay patent 3,298,892 disclose structural sandwich laminates whose core is formed of end grain balsa wood, the laminates having an exceptionally high strength-to-weight ratio as well as excellent thermal insulation properties.
End-grain balsa-cored sandwich laminates are widely used in transportation and handling equipment, such as for floors of railroad cars, shipping containers, cargo pallets, bulkheads, doors, reefer bodies, as well as in a wide variety of other applications. These laminates are also employed for structural insulation in aircraft applications, housing and in boating.
Where the structure to be reinforced is constituted by planar surfaces, the balsa core may simply be a solid board or panel laminated to the facings. But in the case of hulls and other structures having single or double curvatures, or other complex contours, it is ordinarily not possible to conform solid balsa to the contoured surface without bending the balsa panel, and this involves difficult, time-consuming and expensive procedures.
As noted in the Shook U.S. Pat. No. 3,540,967, contourable balsa blankets have been developed that are composed of small individual balsa blocks cut from a board. The blocks are attached to a common carrier, such as a fabric scrim, whereby the blanket may readily be conformed to a curved surface for lamination thereto.
Such blankets, which are commercially available under the trademark "Contourkore," are useful in the construction of reinforced plastic boats and larger vessels, for they lend themselves to lamination between layers of reinforced fiberglass or other plastic material, thereby bringing about a distribution of weight favorable to high stability and buoyancy, as well as imparting stiffness to the structure. While the technique in accordance with the invention produces balsa panels, it is to be understood that these panels may be cut into small blocks or tiles to produce contourable balsa blankets.
As pointed out in the above-identified Kohn et al. and Lippay patents, quite apart from the structural merits of balsa, this wood is of particular value in cryogenic applications, for it has a low coefficient of expansion and hence deforms only slightly under severe temperature changes. Moreover, the k-factor of balsa wood is such as to render this material highly suitable as thermal insulation. The symbol for thermal conductivity is the k-factor, this being the amount of heat expressed in BTU's transmitted in one hour through one square foot of homogeneous material, one inch thick, for each degree of Fahrenheit of temperature difference between opposed surfaces of the material.
If, however, as indicated in the Roberts et al. U.S. Pat. No. 3,894,372, one constructs a system of thermal insulating laminates using a core entirely of balsa board, the cost thereof is quite high. On the other hand, should one make this system with a core of foam plastic thermal insulating material, the cost would then be much lower. But the structural characteristics of the system would be distinctly inferior to balsa; for while foam plastics have acceptable thermal insulating qualities in the cryogenic range, they have poor structural properties.
A factor which the above-identified Roberts et al. patent does not take into account but which, in an era of rising energy costs, now plays a major role in weighing the relative merits of foam plastic material and balsa wood for thermal insulation, is the TOE factor; i.e., "Tons Oil Equivalent."
Foam plastic materials such as polyurethane and polyvinyl chloride (PVC) are petroleum derivatives, and in determining the TOE factor, one must consider the amount of petroleum needed as feedstock for the plastic as well as the amount of petroleum entailed in supplying energy in the form of fuel, electricity or steam to process the plastic material. In the case of balsa, which is a renewable natural product, the TOE is determined mainly by the energy requirements to convert logs into usable board.
Tests indicate that the production of balsa board requires only about 0.150 TOE per 1000 board feet (a board foot is a unit of quantity for lumber equal to the volume of a board 12.times.12.times.1 inches). However, in the case of rigid polyurethane foam of 5 lbs. per cubic foot density, the TOE factor is about 0.565 per 1000 board feet, while in the case of rigid PVC foam of 4.65 lbs. per cubic foot density, the TOE factor is about 0.275 per 1000 board feet.
Hence the TOE factor for balsa production is much lower than for the most widely used synthetic foam plastic materials suitable for thermal insulation. In an era when the conservation of diminishing and non-replaceable petroleum resources is of growing urgency, this distinction is of crucial economic importance.
The reason why foam plastic material is often used in preference to balsa panels as a core material, despite the fact that foam plastic has a much higher TOE factor and is structurally inferior to balsa wood, is that the cost of balsa wood, which is somewhat higher than many foam plastic materials, has heretofore discouraged its use in many industrial and marine applications. One must bear in mind that the cost of a balsa wood product is keyed to the low yield obtainable when employing conventional techniques to convert balsa logs into a usable product.
The traditional conversion technique results in a low yield in that the amount of balsa convertible into usable lumber is usually less than half the total volume of wood in the log. This is primarily due to the constraint that only rectangular or square pieces can be cut from a cylindrical log to produce a final lumber product that has a rectangular form.
In the traditional process, a series of longitudinal cuts are made through the log to produce so-called "flat sawn" pieces whose broad faces lie in a plane parallel to a tangent to the curvature of the cylindrical log. Flat sawn pieces not only give rise to a substantial amount of wood waste, but such pieces tend to warp during the kiln drying process. And even when adequately dried, flat sawn pieces undergo dimensional changes as a result of variations in air moisture or relative humidity, this resulting in deformation of the final product.
As noted in the "Wood Handbook" (Agriculture Handbook No. 72), published by Forest Products Laboratory of the U.S. Dept. of Agriculture (Aug. 1974), the characteristic shrinkage and distortion of flats and squares cut from a log is affected by the direction of the annular rings, tangential shrinkage being about twice as great as radial.
In order to improve the yield obtained from cylindrical logs, it is known to cut logs into interfitting sectors and to join these sectors together to form lumber products. Among U.S. patents which disclose a process for making lumber products in this manner are the Sorensen U.S. Pat. No. 781,376, the Anderson U.S. Pat. No. 2,878,844 and the patents to Hasenwinkle, U.S. Pat. Nos. 3,903,943; 3,961,644 and 3,989,078.
However, none of these prior patents discloses a high yield technique which, when applied to balsa wood, results in balsa wood panels that can be either of the end grain or flat grain type, and which makes it possible to exploit balsa logs in a broad range of diameters running between very young trees having a four-inch diameter and fully mature trees of twenty-inch diameter or greater.
The present invention takes advantage of the fact that balsa trees are fast growing and reach cutting maturity within six to eight years, at which time the diameter at breast high (DBH) can be 10 to 12 inches. Since the invention makes it possible to also exploit young balsa trees of small diameter that are lighter and more readily available than older and larger trees, the invention lends itself to large scale balsa production on ordinary plantations with a very rapid turnover of trees in the order of four to six years.